Japanese Patent Laid-Open Publication No. 2002-2763 64 (JP-A-2002-276364) discloses an example of such a cooling system in which coolant is supplied from an engine cooling system to a motor through a branch passage so as to cool the motor. In this cooling system, a branch portion from which the branch passage extends is provided downstream of a radiator in the engine cooling system such that the coolant is delivered to the motor through the branch passage. After the coolant is further cooled using a radiator for the motor, the coolant is delivered to the motor, and the motor is cooled by the coolant. Then, the coolant is returned to the engine cooling system at a portion downstream of the branch portion. Thus, the engine and the motor are efficiently cooled.
Ordinarily, in a hybrid system including an internal combustion engine and a motor, it is necessary to cool the internal combustion engine, the motor, and a drive circuit for driving the motor (for example, an inverter circuit). A suitable operating temperature range of the internal combustion engine, a suitable operating temperature range of the motor, and a suitable operating temperature range of the drive circuit are different from each other. Accordingly, it is desired to efficiently cool such plural heating elements so that a temperature of each of the heating elements is in the suitable operating temperature range thereof.